The present invention relates to an optical receiving method in which polarization diversity is utilized and a signal lightwave is subjected to heterodyne detection, and an apparatus for carrying out the method. More particularly, the present invention relates to an optical receiving method utilizing polarization diversity (hereinafter referred to as "polarization-diversity optical receiving method"), suitable for realizing an optical receiving apparatus which is high in sensitivity, low in manufacturing cost, and small in size.
In coherent optical transmission in which information is transmitted by utilizing the wave motion of light, there arises a problem that variations in the polarization state of a signal lightwave causes a reduction in signal-to-noise ratio of a received signal. In order to solve this problem, various counterplans have been considered. For example, a polarization-diversity optical receiving method and an apparatus for carrying out this method have been proposed. According to the polarization-diversity optical receiving method, a reduction in power of a received signal due to variations in the polarization state of the signal lightwave is suppressed to prevent a reduction in signal-to-noise ratio of the received signal.
A conventional polarization-diversity optical receiving method and an apparatus for carrying out the conventional method have been discussed in, for example, a paper entitled "POLARIZATION-DIVERSITY RECEIVER FOR HETERODYNE/COHERENT OPTICAL FIBER COMMUNICATIONS" by OKOSHI et al. (the technical digest of the fourth international conference on integrated optics and optical fiber communication held on Jun. 27 to 30, 1983).
In the above method and apparatus, signal lightwave is separated by a polarization separator such as a polarization beam splitter into two polarization components whose polarization planes are perpendicular to each other, and the polarized optical components are combined with two reference lightwaves having the same frequency. Further, two combined lightwaves thus obtained are received by different receivers, and subjected to heterodyne detection, to obtain two detection signals. The detection signals are added to each other, to form an output signal. As a result, even in a case where the polarization state of the signal lightwave is changed so that the signal lightwave becomes linearly-polarized lightwave, and the lightwave component incident on one of the receivers becomes zero, the whole of the linearly-polarized signal lightwave is incident upon the other receiver, and thus the power of received signal is never reduced to zero. That is, it is possible to suppress a reduction in the signal-to-noise ratio of a received signal due to variations in the polarization state of the signal lightwave.
The above method and apparatus, however, have the following drawbacks. That is, part of the signal lightwave is reflected from the polarization separator, and thus the intensity of the signal lightwave incident upon each receiver is lowered. As a result, the sensitivity of an optical receiving apparatus is reduced. Further, owing to the intensity loss at the polarization separator, the signal lightwave is attenuated when the signal lightwave passes through the polarization separator. Thus, the intensity of the signal lightwave incident on each receiver is lowered, and the sensitivity of the optical receiving apparatus is reduced. Further, two optical couplers are required for forming two combined lightwaves. This increases the manufacturing cost of the optical receiving apparatus. In order for each combined lightwave to be subjected to heterodyne detection, the optical receiving apparatus is required to include a pair of receivers; a pair of electric circuits, each of which is connected to a corresponding one of the receivers and includes a power source; an amplifier; and a pair of optical elements such as lenses. Thus, the manufacturing cost of the optical receiving apparatus is increased. Since the optical receiving apparatus includes a pair of receivers and electric circuits, the power consumption of the optical receiving apparatus is increased, and hence the operating cost thereof becomes high. Since the optical receiving apparatus is high in power consumption, an expense necessary for cooling the apparatus is increased. Further, in order to make the characteristics of two receivers substantially equal to each other, it is necessary to select two receivers having the same characteristics from a multiplicity of receivers. Thus, the manufacturing cost of the optical receiving apparatus is increased.